The present invention relates to a method and a device for adjusting the braking effect in a motor vehicle.
In the related art there are various known options for counteracting the tendency of a braked wheel to lock-up, for example, by manipulating the braking force on each wheel using individual control. However, for such individual control, a method of controlling braking effect on each wheel individually, i.e., separately from the other wheels should be provided. In the case of hydraulic brakes, such systems provide a control channel with the corresponding hydraulic components for each wheel brake to be controlled individually. Reference is made here to
Bosch Technische Berichte, Vol. 7, No. 2, 1980 for anti-lock systems as an example.
In addition to such brake systems, however, it is also known that the braking effect on multiple wheels of a motor vehicle can be regulated or controlled jointly. German Patent No. 22 43 260 (corresponding to U.S. Pat. No. 3,907,378), for example, describes how the braking effect on the wheel brakes of the wheels of one axle can be adjusted jointly in the sense of preventing brake locking. Since the wheels on one axle may be traveling on parts of the road surface having extremely different road surface/tire coefficients of friction, it must be determined which of the wheels that are to be controlled jointly determines the braking effect. In this connection, it is known that two operating modes can be provided for jointly setting the braking effect on two wheels of one axle.
In one operating mode, the select low mode, joint setting of the braking effect, i.e., the brake pressure in general, is based on the wheel with the lower road surface/tire coefficient of friction. Thus, in the select low mode on one axle, the brake pressure on the wheel brakes of this axle is set so that the low wheel, i.e., the wheel with the lower road surface/tire coefficient of friction of the two wheels, is operated at optimum slip. Consequently, the high wheel, i.e., the wheel with the higher road surface/tire coefficient of friction, is underbraked and is stable (no excessive wheel slip), but then it can also withstand high lateral traction forces.
In the select high mode, the joint setting of the braking effect, or, in general, the brake pressure, is activated according to the wheel with the higher road surface/tire coefficient of friction. Thus, in the select high mode on one axle, the brake pressure on the wheel brakes of this axle is set so that the wheel with the higher road surface/tire coefficient of friction of the two wheels is operated at optimum slip. Consequently, the wheel with the lower road surface/tire coefficient of friction is overbraked and may even lock-up and then can withstand little or no lateral traction forces. The advantage of the select high mode, however, is that the vehicle has a relatively short braking distance.
German Patent No. 22 43 260 describes how the period of time during which the wheel is switched to select high mode is to be made to depend on the vehicle speed and/or deceleration so that the time is longer at a lower vehicle speed and/or deceleration.
Likewise, U.S. Pat. No. 3,535,004, which is cited in German Patent No. 22 43 260, describes how switching to select high mode is to be performed whenever the vehicle is traveling on a road surface with a coefficient of friction that differs significantly on the right and left sides of the vehicle.
In addition, it is known that with a two-channel ABS (braking effects on the front and rear axles are regulated jointly) or a three-channel ABS (braking effects on the rear axle are regulated jointly, but an individual setting of the braking effect is possible on the front axle), the braking effect on the rear axle is set for select low mode,.
The object of the present invention is to provide for a brake system with a joint setting of the braking effect on the wheels of at least one axle so as to achieve the greatest possible driving safety.
The present invention concerns a system for setting the braking effect in a vehicle in the sense of avoiding the tendency to lock-up, with at least two wheels which are arranged on an axle on the right and left sides with respect to the direction of travel and which travel on a road surface having different coefficients of friction, and whose motion characteristics are detected. The braking effect on the two wheels is set jointly, with two operating modes being provided, which can be selected depending on the prevailing coefficients of friction on the wheels and/or depending on the vehicle speed. In the first selectable operating mode (select low), the braking effect is jointly set depending on the motion characteristics of the wheel having the lower coefficient of friction, while in a second selectable operating mode (select high) the joint setting is based on the motion characteristics of the wheel having the higher coefficient of friction. According to present invention, the coefficients of friction of the wheels on one axle are determined, and then the difference between the coefficients of friction on the wheels on the right and left sides of the axle with respect to the direction of travel is determined. When differences between the coefficients of friction reach a predeterminable lower value, an operating mode is selected according to the prevailing coefficients of friction. As an alternative or in addition, an operating mode may be selected according to the detected vehicle speed when the coefficient of friction differences reach a predeterminable greater value.
Thus, the difference in instantaneous coefficients of friction of the road surface on the right and left sides of the vehicle is determined. If this difference is below a predetermined threshold, an operating mode is selected according to the prevailing coefficients of friction. As an alternative or in addition, an operating mode may be selected according to the detected vehicle speed when the threshold is exceeded.
Especially when used in a two-channel brake system, the present invention yields a brake system with a simple but inexpensive design that increases driving safety.
In particular, the present invention provides for the second operating mode (select high) to be chosen, in particular for the front axle of the vehicle, when there are low coefficients of friction and the coefficient of friction difference is small. With this exemplary of the present invention, locking of wheels, in particular a front wheel, is acceptable under some circumstances in order to achieve a short braking distance. However, the rear wheels should advantageously be operated in select low mode to ensure optimum lateral traction. In principle, however, the rear wheels can also be operated in the select high mode.
The first operating mode (select low) is advantageously selected, in particular for the front axle of the vehicle, when the coefficients of friction are high and the coefficient of friction difference is small. Because of the high coefficients of friction, good lateral traction of the wheels should not be sacrificed in this case.
Another advantageous exemplary calls for the second operating mode (select high) to be selected, in particular for the front axle of the vehicle, when the vehicle speed is low and the coefficient of friction difference is high. When vehicle speed is high and the coefficient of friction difference is great, the first operating mode (select low) is selected, in particular for the front axle of the vehicle. This exemplary is based on the consideration that at lower vehicle speeds, a non-optimal lateral traction of the front wheel with the lower coefficient of friction can be accepted under some circumstances to optimize the braking distance. At higher vehicle speeds, however, the select low mode should be set on the front wheels to increase the lateral traction of the front wheels and thus improve steerability. In this exemplary, however, the rear wheels should also be operated in the select low mode regardless of vehicle speed to ensure optimum lateral traction of the rear wheels and thus stable vehicle performance. In principle, however, the rear wheels may also be operated in the select high mode.
Coefficient of friction differences of a larger and/or smaller value can be detected by
detecting the rotational motion of each wheel,
determining the coefficients of friction on the wheels depending at least on the detected wheel motion,
comparing the difference between the coefficients of friction determined on the right and left wheels with preselectable threshold values.
The wheel deceleration is determined in this case from the rotational motion thus detected. The coefficient of friction can be deduced from the wheel deceleration, in particular the maximum wheel deceleration.
One advantageous exemplary of the present invention calls for the second operating mode (select high) to be selected in particular for both axles of the vehicle when coefficient of friction differences are very great. This exemplary is based on the consideration that unacceptably long braking distances can occur due to the select low mode when coefficient of friction differences are extremely different.
The selection of an operating mode can be provided with a hysteresis according to the present invention, so that there are not too many switches (pendulum switches) between operating modes at low coefficient of friction differences.